Single-conversion UPS systems are those in which, during normal operation, the incoming line is used to provide power to the critical load either through a transformer or in conjunction with some series impedance.

Some forms of single-conversion UPS products are classified as “line interactive”. The single conversion UPS usually provides a higher operating efficiency at lower cost than the double conversion UPS at a comparable system reliability. (Note that the recently revised NEMA PE 1-1993 [B23], identifies the single-conversion system as a “single-conversion converter.”)

Unlike the double-conversion system, the incoming line to the single-conversion UPS is not rectified to produce dc power to provide input to the inverter. The normal ac power is supplied directly to the critical load through a series inductor or a linear or ferroresonant transformer.

The normal ac also supplies a small charger used to maintain the UPS batteries in a fully charged condition. Thus the battery is only used when inverter requires the battery’s output to supplement or replace the normal power source. Single-conversion UPSs include the ferroresonant type, some variants of the tri-port type, line interactive types, and some SPS designs. A discussion of some of these systems follows.

Line interactive systems
The line interactive UPS is rapidly achieving prominence in the industry. Lower cost, simplicity of design, and smaller “package” are some of the advantages of this design.

In the line interactive UPS, line power is not converted into dc but is fed directly to the critical load through a series inductor or transformer. Regulation and continuous power to the load is achieved through use of inverter switching elements in combination with inverter magnetic components such as inductors, linear transformers, or ferroresonant transformers.

The term “line interactive UPS” derives from the fact that the inverter interacts with the line to buck, boost, or replace incoming power as needed to maintain constant voltage to the critical load. In the absence of line power, the line interactive UPS provides the total power to the critical load.

Some line interactive products exhibit a break in power to the critical load during line-to-battery or battery-to-line transfers (this type of system would be an SPS). Other designs are capable of producing true unbroken power to the critical load. Line interactive UPSs are available as single-phase or three-phase units.

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